The present invention relates to the creation of optical networks and, in particular, discloses various algorithms to optimise the placement of optical elements within a network.
Recently, the construction of optical networks has become more and more significant for forming the basis for data transmission. Optical fibers have been found to have higher and higher levels of useable capacity and hence they are beginning to dominate the area of long haul data transmission.
The design of an optical fiber network is often an extremely complex process. The layout often includes very complex components that must be placed at certain geographical locations so as to meet overall performance requirements. Often, it is desirable to minimise the number of components in a network so that over engineering is avoided.
As the creation of an optical network can be an extremely complex and expensive process, there has risen a general need for the accurate simulation of optical networks utilizing software based systems. Simulations allow a network design to be xe2x80x9ctrialedxe2x80x9d in a virtual environment, which is much faster and less expensive that constructing an experimental prototype, laboratory or field trial system. However, while simulation is a very cost effective method to verify an existing design, by itself simulation does not filly address the overall problem of system synthesis.
Currently, the initial design (or synthesis) of an optical transmission system is a complex task that is best carried out via the intensive attention of an expert human designer. It is highly desirable to be able to capture equivalent expertise within a software synthesis system, to enable optical transmission systems to be preferably designed by people lacking such high levels of expertise.
The process of synthesis involves taking an initial set of design constraints and objectives, and generating a system design using equipment selected from a specified range of suitable, commercially-available components. The design constraints typically include such factors as existing installed fiber plant and geographical facility locations that must be used. The design objectives typically consist of overall performance and cost requirements.
Such a software synthesis system allows for complex planning to be undertaken, including the testing of multiple alternatives without having to place or alter complex and expensive structures in the field.
It is an object of the present invention to provide for an improved form of optical network synthesis and planning.
In accordance with a first aspect of the present invention there is provided a method of calculating an equivalent fiber representation of a multiplicity of actual fibers, the method comprising the steps of successively combining the parameters of an initial pair of adjacent fibers to compute the parameters of a third fiber that has substantially the same transmission properties as the two original fibers, and wherein one of the initial fibers may itself be an equivalent fiber representation. Alternatively, the method may comprise the step of combining the parameters of a multiplicity of fibers, comprising an arbitrary combination of actual fibers and equivalent fiber representations, to compute the parameters of a single replacement fiber that has the same transmission properties as the multiplicity of fibers.
In accordance with a second aspect of the present invention, there is provided a method for optimizing the topology of an optical data transmission link, the link comprising a series of optical components interconnected by fiber sections, the method comprising the steps of:
a) calculating an equivalent fiber representation of the link;
b) eliminating redundant optical components from the link;
Preferably the method also contains the step of:
c) generating solutions utilizing components selected from a set of available devices;
Preferably the method also contains the step of:
d) utilising an optimisation procedure to determine the input channel power of the link.
The step (a) preferably can comprise the steps of: (a1) progressively joining segments together between facilities and determining whether an equivalent fiber representation for the joined segment falls within predetermined constraints; (a2) progressively joining segments together across facility locations and determining whether an equivalent fiber representation for the joined segment falls within predetermined constraints.
The step (b) can comprise the steps of: (b1) calculating a measure of the signal quality required at the receiver to satisfy the link performance criteria; (b2) calculating the maximum acceptable transmission power for said link; (b3) determining suitable configuration values for predetermined components in the link. The configuration values can, for example, include the amplification factor of an optical amplifier, and the total dispersion of a dispersion compensating module.
The step (b1) can further comprise the step of calculating a numerical measure of signal quality corresponding to a specified link performance. The measure of link performance can, for example, be the probability of transmission errors occurring on the link. The numerical measure of signal quality can be, for example, the Q-factor. The step (b1) can also comprise the step of adjusting this signal quality measure to account for additional sources of degradation in transmission. The additional sources of degradation can be, for example, nonlinear transmission processes such as four-wave mixing. The step (b1) can further comprise the step of calculating a simplified measure of signal quality at the receiver that is more easily optimised in the subsequent steps. The simplified measure of signal quality can be, for example, the optical signal-to-noise ratio (OSNR).
The step (b2) can further comprise the step of calculating the maximum transmission power that will not result in unacceptable signal degradation due to nonlinear transmission effects and additive noise. The nonlinear transmission effects considered can include, for example, four-wave mixing and stimulated Raman scattering. The additive noise considered can be, for example, amplified spontaneous emission noise introduced in optical amplifiers. The step (b2) can further comprise the step of verifying that the output power is valid, within the operating ranges of components in the link. This can include, for example, verifying that the transmitters are capable of generating the required output power.
The step (b3) preferably includes the step of determining the amount of residual dispersion required to optimise the signal quality at the receiver, considering the impact of nonlinear propagation effects. The nonlinear propagation effects can be, for example, the self phase modulation of each transmitted channel. The step (b3) preferably further includes the step of applying dispersion compensation to achieve the required residual dispersion by selecting from available dispersion compensating modules. The available dispersion compensating modules can include, for example, lengths of dispersion compensating fiber. The step (b3) can also include the step of compensating for the insertion loss of the dispersion compensating modules by including further optical amplifying units in the link.
The step (c) preferably includes the step of: (c1) replacing generic components generated in the previous steps with actual components selected from available devices. The generic components to be replaced include, for example, the optical amplifying units placed in the link. The step (c) preferably further includes the step of generating multiple solutions using the available devices, that can be ranked according to additional criteria. The additional criteria can be, for example, the total cost of the link. The step (c) can also include the step of determining the powers of all signal channels at the receiver, and inserting a receiver preamplifier if required to make the signals compatible with the receiver. The requirements for compatibility can include, for example, sufficient received power and sufficient received signal-to-noise ratio.
The step (d) preferably can include the step of optimising the pre-amplification gain and input channel power for the link. Further, this can include the step of: equalising the powers of input channels to obtain a substantially flat spectrum across the transmission bandwidth.